RIEDEL MEGASHEARS R' AND THE TREND TO GRAVITATIONAL EQUILIBRIUM AS MAIN FACTORS OF TSUNAMIGENIC EARTHQUAKES

An earthquake source is characterized by two nodal planes oriented parallel to two planes of maximum shear stresses (Fig. 1, left). A rapid displacement of the shear type (in mechanical, rather than in the geological meaning) occurs along one of the planes and causes an earthquake.

The concept of plate tectonics with one of its main components, subduction zones, provides, at first sight, the unique opportunity to select one of the two nodal planes – a gently dipping plane which is parallel to the roof of the subducting oceanic plate (Fig. 1, bottom right). The other nodal plane that is steeply dipping in the opposite direction (Fig. 1, top right) seems ‘unpromising’, considering the aspect of seismicity, for two reasons. First, displacement along this plate is contrary to the general direction of oceanic plate subduction. Secondly, such displacement is directed against the direction of gravity, which is energetically disadvantageous.

However, it should be taken into account that in the stress field of the subduction zone, as in any stress field, the two above-mentioned maximum shear stresses have equal values. At the same time, it is the sub-vertical displacement that excites rapid uplifting of the seabed which causes a tsunami. Researchers who support the traditional choice of a gently dipping nodal plane have to reckon with it and therefore create complex models, such as the ‘splay fault’ model that seem most successful, though being quite complicated and controversial (Figs. 56 and 57).

In our opinion, the geological reality is more adequately refelected by the geological and geophysical model shown in Fig. 1 (right). It is based on the wide range of information and assumes that both nodal planes are equivalent and interchange in generation of strong earthquakes.

The aim of this article is to consider this model in terms of tectonophysics. For this purpose, earthquake sources indicated on (Fig. 1, right) are classified as Riedel megashears, R (bottom right) and R' (top right top), which occur in the geodynamic setting of sub-horizontal shearing (in this case, subduction of the oceanic plate) along the sub-horizontal plane (Fig. 3). This situation is one of five elementary geodynamic settings (see Fig. 2). It is similar in everything, except the position of the shearing plane, with the geodynamic setting of horizontal shearing along the vertical plane (Fig. 4). Riedel shears formed in the latter situation were subject to the most detailed studies using purpose-made devices (Fig. 5, and 6). This study gave grounds to conclude that Riedel shears, R are developed much better than shears R'.

Our experiments (Fig. 7) confirm the above conclsuion. Moreover, it is revealed that shears R', that develop poorly in samples made of wet clay (Figs. 8, 9, 12, and 13), cannot develop in a granulated medium such as a mixture of sand and solid oil (Fig. 10, 11, and 14) and do not develop in other granulated media (Fig. 17), which are similar to the block structure of the uppermost crust (Fig. 18–20). In such mediums, shears R result from joining of small echeloned tension joints. Such style of shear formation has been explained in various waysare proposed (Fig. 15–16), and the main point of the explanations is joining of small tensile fractures by means of larger shear fractures. However, our experiments with wet clay (Fig. 31–35) show that even artificially created ’Riedel shears’ show nearly a zero extension under loading followed by shearing, which casts doubt on possibile occurence ofshear fractures as such without involvement of smaller tenson joints.

While being not satisfied with the results of our experiments, we carried out numerical simulations of the evolution of Riedel shears, R and R' for different values of lithostatic pressure (which is actually impossible in experiments with equivalent materials) and angles of shearing. (See Fig. 41 for real values of lithostatic pressure and tangential stress with reference to depths of tsunamigenic earthquakes). The opinion voiced by several authors was confirmed – the effect of unequal rotation of the shears during the subsequent shearing is highly significant and therefore ‘subversive’ for shears R'. This simulation was carried out under the assumption of emerging of shears without participation of smaller tension joints (although this assumption is not consistent with the results of our experiments, see above) (Fig. 21–30). Numerical simulation was problematic for the case involving tension joints and had to bereplaced by experiments with thephysical modelwhere small tension joints were artificially created and arranged in an echelon pattern along the tracks of future shear fractures, and small joints and tracks were oriented in accordance with the orientation of the vector of principal stresses that occurred in the model made of wet clay due to shearing (Fig. 36–40).

The results of both physical and numerical modeling have led to a definite conclusion that Riedel shears R are evidently dominating over shears R' in a variety of conditions (except for the initial stages of shearing in the samples of wet clay, which, by virtue of internal connections between clay particles, gives a less adequate representation of the natural block-type geological medium than granular materials).

This conclusion is in contradiction with the well-justified model combining geological and geophysical indicators of the formation of foci of strong tsunamigenic and non-tsunamigenic earthquakes (see Fig. 1) which are identified (see above) as megashears R and R', respectively. This contradiction is eliminated if we take into account the sharp gravitational disbalance of the island arc – trench ‘tectonopair’ created by subduction. This disbalance is expressed in the contrasting relief and in contrasting gravity anomalies in this ‘tektonopair’ (Fig. 43). We assumed that nature cannot be ‘tolerant’ for a long time, and found an opposite natural reaction (mainly in the case of the Tohoku earthquake in Japan on March 11, 2011) – subsidense of the Earth surface segment adjacent to the island arc and uplift of the surface segment adjacent to the trench, accompanied by horizontal movement of the material from the arc towards the trench (Figs. 47–54, and 58). This process has a trend of declining relief contrast between the arc and the trough and inversion of the sign of gravity anomalies (Figs. 44–46). And it is the boundary between these regions of the Earth surface subsidence and uplifting, to which tsunamigenic earthquake are confined at reverse faults of the seabed surface with the raised wall facing the trough (Fig. 42). This means that the tendency to gravitational equilibrium realized the potential of forming megashears R', that develop much worse than shears R (or do not develop at all) in other natural and modelled settings.

The conclusion that foci of tsunamigenic earthquakes R' are confined to the margin between sibsiding and uplifting regions challenges the traditional concept that a tsunami is a consequence of a sharp rise in the seabed in the local uplift area. A slashing subsidence of a vast area of the seabed entails an equally sudden sharp lowering of the sea level and the retreat of the sea from the coast. Such a phenomena was observed by unlucky tourists at the Phuket island just before the Sumatra tsunami. In a similar way, a sudden uplifting of the seabed in the area adjacent to the trough causes a corresponding rise of the sea level. In such cases, masses of water, that are much more mobile than terrestrial masses, are subject to the gravitational disequilibrium, rush towards the shore and cause a tsunami (Fig. 55).

A consolidated model of tsunamigenic earthquakes resulting from the trend to restoration of the gravity equilibrium is shown in Fig. 63. According to our conclusions, it is recommended that tsunamigenic earthquakes forecasting should be based on continuous high-precision and high-frequency monitoring of GPS and gravitational field measurements and aimed at early detection of a tendency to inversion of tectonic movements and gravity anomalies in the island arc – trench ‘tectonopairs’.

Observations of the so-called seismic ‘nails’ (Figs. 59–61) should also be conducted. Seismic ‘nails’ can be interpreted as incipient Riedel megashears R', consisting of smaller tension megafractures (similar to those shown in Figs. 10, 11, 14, and 17), which are viewed as precursors of a strong earthquake.

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